As shown in U.S. Pat. No. 4,157,996, assigned to the same assignee as the present invention, a solution of aromatic bis-(ether dicarboxylic acid), an organic diamine and a monoalkyl ether of ethylene glycol or diethylene glycol is coated on a substrate and polymerized to form a high quality polymeric film. However, experience has shown that such mixtures often cannot tolerate appreciable amounts of water, for example, above about 1.0% by weight in instances where monoalkyl ether of ethylene glycol or diethylene glycol is utilized as the solvent without precipitation occurring. The wire coating industry is continuously investigating the formulation of wire coating solutions which can utilize inexpensive monoalkyl ethers of ethylene glycol and of diethylene glycol as a principal solvent for high solids enamels in combination with non-precipitating amounts of water, for example, up to about 10% by weight of the mixture to reduce overall manufacturing costs while satisfying environmental requirements and giving improved enamel behavior.
The present invention is based on the discovery that organic dianhydride of the formula, ##STR1## where R is a C.sub.(6-30) aromatic organic radical, can be heated with monoalkyl ether of ethylene glycol or diethylene glycol in the presence of significant amounts of water, for example, as high as 10% by weight of the mixture to produce a clear solution comprising a mixture of the corresponding tetra-acid of the organic dianhydride of formula (1) and the half-ester resulting from the reaction of the monoalkyl glycol ether and the organic dianhydride. There then can be added to the clear solution, at least equal molar amounts and including up to about 25 mole percent excess of organic diamine of the formula, EQU H.sub.2 NR.sup.1 NH.sub.2, (2)
based on total moles of initial organic dianhydride of formula (1) utilized, where R.sup.1 is a C.sub.(6-30) organic radical. The resulting formulation can be utilized as a clear, stable wire coating solution which can remain unchanged at ambient temperatures for an indefinite period of time and thereafter used as a coating composition, for example, a wire coating enamel to produce flexible films under conventional wire coating conditions.